Same here, I have been looking into modifying the max fleet size, as I think ships should be limited by the overall points only (not 5 capital ship max).

I do not think it can be changed directly.
A possible workaround is to create dreads/carriers templates and classify them as "cruisers".

I found out a while ago that this is hard coded and cannot be modded

Would it be possible to change the number battleships take up from 1 to 0.5? As in not raising the maximum, just being able to have twice as many battleships compared to dreads or carriers?

Nope. If it were possible, you could just put all supercap to 0.

The only thing I have found is to add ships to the fleets through the save file. It should work, seeing as exceeding the maximum is possible. At least it happened to me in the Titans Dlc where I suddenly had something like 30 capital ships, that the game just gave me. Could be a glitch, but it's worth a shot, even if it's a bit annoying to add them as a text, but if you really want a bigger fleet, I'd give it try.